CAUSES OF CLIMATIC CHANGES. 325 



The conclusion that volcanic dust is secondary to changes in solar radiation 

 in producing climatic changes seems warranted by the facts, but it is probable 

 that its value is greater than that assigned by Huntington, and less than that 

 assumed by Humphreys. The temperature departures for 1767-1770, 1785- 

 1788, 1816, and 1837 are much greater than during the past 30 years. More- 

 over, the conditions during the middle of the fourteenth century indicate 

 that the major sun-spot cycle pointed out by Huntington (1914^: 552) was 

 supplemented by the screening action of volcanic dust. It is interesting, if 

 not significant, that the Jacquerie of 1358 followed several years of famine 

 which appear to have coincided with the maximum of the major cycle. It is 

 not improbable that both the Jacquerie and the French Eevolution were the 

 human sequences of crop failures and climatic changes initiated by sim-spot 

 maxima, and emphasized by volcanic eruptions. 



The solar hypothesis. — ^The primary importance of the solar h3rpothesis in 

 explaining present and past changes of climate is emphasized by Huntington 

 (1914": 477). His presentation is so detailed and complete that only a brief 

 smnmary is possible here. The original paper must be consulted by those 

 who wish to judge for themselves of the value of his convincing assumptions. 

 The discussion falls into three major portions, the last of which will be sum- 

 marized under Permian and Pleistocene glaciation. These are : (1) hypotheses 

 in explanation of present climatic changes, (2) the climate of historic times, 

 (3) the coimection between historic changes of climate and the glacial period. 

 The hypotheses discussed are : (1) the meteorological, (2) the volcanic, (3) the 

 solar. The importance of the first in producing changes which are observed 

 from year to year is admitted, but it is held that it does not explain the larger 

 annual variations. This is indicated by the fact that the vast majority of 

 such variations in temperate regions are due to the number and location of 

 cyclonic storms, which seem to vary markedly in harmony with sun-spots. 

 Moreover, meteorological accidents should not occtir regularly, and yet the 

 work of Arctowski upon pleions and antipleions of temperatm-e shows that 

 they do. Huntington's conclusion as to the volcanic hypothesis, namely, 

 that it is of real but secondary importance, has already been noted. 



Disagreement between solar and terrestrial temperature changes is indicated 

 by Hmnphreys's figure of solar heat, sim-spots, terrestrial temperatures, and 

 volcanic eruptions, as pointed out by Huntington (485). The actual measure- 

 ments of radiation received from the sun do not agree with the earth's tempera- 

 tures, and the greater similarity of the P+S curve, rather than the sun-spot 

 cm^^e, to the temperature curve, indicates the presence of some other factor. 

 This is supported by the fact that the solar constant is higher during sun-spot 

 maxima than during minima, while the terrestrial temperatures are just the 

 reverse. This suggests that some other factor than the increased temperature 

 of the sun's surface is at work modifying the earth's temperatures. This 

 factor is the wind, which seems to be the most important in producing varia- 

 tions in terrestrial temperature, since variations in the sun's temperature 

 seem too small to play an appreciable part. While Newcomb's conclusion 

 that there is a consistent fluctuation of 0.47° F. between sun-spot mayiTniiTTi 

 and minimum is accepted, his further conclusion that "all the ordinary 

 phenomena of temperature, rainfaU, and winds are due to purely terrestrial 

 phenomena, and .that no changes occur in the sun's radiation which have any 



